1. Background of the Invention
The present invention generally relates to medical electrical stimulation/sensing electrodes. More specifically, the invention is concerned with a large conductive area electrode located subcostally outside the chest cavity and utilized in connection with other electrodes in an implantable cardioverter/defibrillator (ICD or AICD) system for delivering an electrical shock to the heart. These other electrodes may be located externally or internally with respect to the vascular system.
2. Description of the Prior Art
An electrode with a large conductive area located external to the heart is commonly referred to as a patch electrode in the art. As is known, a patch electrode may be of various forms and shapes, i.e., conductive disk or a planar or cup-like conductive mesh. However, when used in its broad sense, the term patch electrode identifies an electrode distinct from an intravascular type electrode, such as a helical or braid electrode.
Cardioverter/defibrillator systems may employ a variety of electrode configurations, such as the entirely epicardial patch electrode system disclosed in U.S. Pat. No. 4,030,509 or the entirely intravascular electrode system disclosed in U.S. Pat. No. 3,942,536. The entirely intravascular catheter electrode arrangement requires relatively high energies to defibrillate the heart and is, even with high energies, not capable of defibrillating the heart in some patients. Thus, while such a system avoids the necessity of surgically opening the chest cavity to implant the electrodes, as is required with epicardially located electrodes requiring relatively low defibrillating energy, an additional patch electrode must be used to reduce the energy required for defibrillation. In order to leave the chest cavity intact, this additional patch electrode is commonly located subcutaneously, outside the rib cage.
One example of an electrode configuration in which a patch electrode is combined with intravascular electrodes is disclosed in U.S. Pat. No. 4,662,377. In this known arrangement, a flexible, subcutaneously placed patch electrode (positioned between the rib cage and the skin) is electrically connected to a first intravascular electrode in the superior vena cava region, both of these electrodes forming one pole in the shock delivery circuit, and a further intravascular electrode is located in the right ventricle of the heart, forming the other pole.
As already mentioned, patch electrodes of different types have been proposed. U.S. Pat. No. Re 27,757 discloses an early attempt to combine an intravascular electrode and a patch electrode. The patch electrode described therein has a flat plate placed epicardially, sutured under the skin of the anterior chest wall or placed on the surface of the chest. However, there is no teaching in this publication as to how a flat plate sutured under the skin, or even epicardially placed, might be constructed to withstand the movements of the body at such electrode locations over a long period of time.
In contrast, in current and practically used epicardial as well as subcutaneous patch electrodes, flexibility is recognized to be essential. Typically, therefore, they consist of a metallic mesh surface on the side of the electrode facing the heart and an electrically insulating silicon backing on the other side in order that the electrode be flexible enough to accommodate the "wringing" action of the heart and the stretching and contracting actions of the skin, respectively.